1. Field of the Invention
The invention relates to an operational amplifier, and more particularly, to a source driver of display that includes an operational amplifier having an adjustable bias current.
2. Description of the Prior Art
Liquid crystal displays (LCDs) have led us to a brave new visual world for their small size, light weight, and extensive display capabilities. One important subject in evaluating the display capability of an LCD is its response time. An LCD having a shorter response time can clearly display fast-moving objects, whereas an LCD having a longer response time would create a smear or blur pattern around moving objects, making them unacceptable for viewing moving video. To improve the response time of the LCD, an important issue is to improve the driving capability of the LCD's source driver. As known by people skilled in the art, the LCD's source driver drives the LCD by charging each pixel of the LCD to a corresponding voltage level. A bottleneck in improving the driving capability is the charging time of each pixel being dominated by the pixels' capacitive nature.
Certain conventional techniques have been developed to improve LCD response times. With regards to the driving current, FIG. 1 shows an operational amplifier (OP-AMP) at the source driver's output stage, which has an adjustable bias current. The OP-AMP 100 generates an output Vout according to an input Vin, and is implemented using transistors M1-M5, capacitor C, and current sources I0, I1, and I2. While the OP-AMP 100 is in a steady mode, it is normally biased by a first current source I1. When the OP-AMP 100 is in a driving mode, the switch SW is turned on to boost the bias current of OP-AMP 100 by adding a second current source I2. The driving capability and slew rate of the OP-AMP 100 will hence be improved a great deal at the driving mode. The steady mode can be defined as a period that the voltage of input Vin of the OP-AMP 100 is not changing (steady). The driving mode can be defined as a period that the voltage of the input Vin of the OP-AMP 100 is changing (transition). This technique, however, has a drawback of causing a sudden bias current change, resulting in output waveform glitches. The glitches in the output waveform will trigger bouncing and cause mura on the LCD, significantly degrading the display quality.